[BITV/A11Y] Accessibility improvements

[st3iny]

The accessibility of the calendar app should be improved. Here is a list of things that could be done.

• Calendar visibility toggle (entry itself) is not focusable via keyboard #3955 (important)
• Calendar content area is not accessible/focusable #4123 (important)
• Clicking on New event button always switches to month view #3522 (important)
• "+ New event", the + should be an icon and not in text #3945
• Accessibility: "+ New Calendar", the + should be an icon and not in text #3965
• Remove aria-label and title of "Today" button, so that just "Today" is read as it should be #3934
• Color indicator circle of disabled calendars is almost invisible #3966
• Relationship between events and calendars relies on colors #4128
• Wrong focus after closing "New calendar" #4127
• Add headings to appointments #4126
• "New calendar" flow:
  • "New calendar" has 2 tab stops, 1 link and 1 button – should just be 1 button #4029
  • Title input field misses a label #3932
  • "New calendar" submit icon is not focusable #4125
• Redesign calendar sharing #4124

[tcitworld]

FullCalendar 5.10 seems to have a11y improvements https://github.com/fullcalendar/fullcalendar/blob/master/CHANGELOG.md#v5100-2021-10-13

[jancborchardt]

Sorted the most important 3 (bold) issues on top – as per judging from the notes document.